By Steve Gerbig, DSM Engineering Plastics
There are several variables that impact the long-term success of a new program. Some of these variables include material, processing, tooling, and manufacturing. Any one of these may affect the manufacturing efficiency of the product that has a direct effect on profitability. Although there are many causes for production inefficiency, there seems to be one variable that stands out above all the rest. That variable is tooling. Often molders will say they cannot process materials or mold acceptable parts due to limitations created by the mold. Unless these problems are corrected during the development stage, the molder is forced to make a product at less than quoted standards. The customer receives a product below expected quality levels and in the end everyone loses. To eliminate or reduce these problems it is imperative the molder, tool-maker, and product designers work together during the development stage to resolve as many of these issues as possible. The following items are important to note in obtaining success with a new mold:
Plastic Materials
First and foremost, have an understanding of the material required for the program. Amorphous and semi-crystalline materials have different properties, melt, and flow characteristics. If the part design calls for long flow paths or deep ribbing, extra part thickness may be required to ensure filling. If the material has fillers or reinforcements it will affect tooling life and proper steel selection is critical. Discuss possible material changes in the future for cost saving programs. This may have an influence on tooling materials selected for mold construction. Remember, plastic materials are not created equal. Amorphous and semi-crystalline materials have their own processing characteristics and behavior. Don’t expect these materials to process the same in a given mold. Guidelines for the correct melt and mold temperatures need to be used to ensure part quality and good physical/mechanical properties of the finished product.
Steel Selection
Steel selection is extremely important. Knowing and understanding the types available, one should be able to select the correct steel for the resin selected. When a molder incorrectly selects a mold steel, for example, if they do not realize a flame retardant package is in the material, the result can be mold corrosion and excessive maintenance costs. For use with flame retardant materials, a medium to hard stainless steel (CSM- 21 or 420SS), or a plating such as chrome or nickel be used to prevent steel corrosion. If plating is selected, remember there is a cost associated with stripping and re-plating if revision work or other engineering changes are made. For glass filled materials use hardened H-13 or S-7.
Mold Construction
There are a few items that will help to improve part moldability and eliminate costly downtime. Parting line locks are imperative for correct alignment of the core and cavity halves and to keep the halves from moving when pressure is exerted during mold filling. Often you see molds in the field that have none. In those cases, the leader pins are relied upon to keep the halves aligned. Wear will eventually cause parting line mismatch. For large molds with deep cavities it is best to incorporate the locks in the sides of the mold base and to use S7 wear plates on one half and H13 on the other to prevent galling. Shim stock can be used if necessary if the fit between the halves needs to be tightened. On smaller molds standard bar locks can be purchased from suppliers of mold base accessories. For deep draw parts that have undercuts utilize ejector lifts. This eliminates the need for ejector pins that wear and over time have to be replaced. Venting can also be added below the lift so air and volatiles can escape or to ensure parts are completely filled. When incorporating lifts in molds make sure they are pinned and bushed for ease of movement and location. Once a preliminary set of mold drawings are made, discuss gate and runner size, cooling channel size and amount of circuits to be incorporated. Also, discuss any potential or future engineering changes that might have to be made. This will allow you to determine what areas in the mold will be affected and what can be done to incorporate the future change with the least amount of downtime and costs. This is the right time to find out if pillar blocks, water lines or core pins are in the way.
Mold Safeties
There are occasions where the design of a part requires an ejector pin to be out of the way before a side core is allowed to advance to its closed position. If a redesign of the product is not possible, it may be necessary to move the cam hydraulically with proper timing and safety interlocks. Positive ejection return should also be used to ensure the ejector plate is in the back position before the mold is allowed to close. In cases where an extra plate is utilized for a three plate mechanism, safeties should be installed to lock the plate into position before the mold is closed. An example of this is a mold that incorporates lifts and pins, whereby the lifts and pins travel the same distance until all undercuts are cleared and the extra plate moves the pins out further for complete ejection. Any significant safety requirement should be detected in the design phase, otherwise costly damage is inevitable. The development stage of the mold is where any “gnats” creating molding problems, part quality, or efficiency standards need to be addressed. In most cases, once a molder is awarded a job he will have that mold for several years. Unless these issues are resolved early on it will become a day-to-day battle to produce a product that will make a profit. With this in mind, a well-built reliable mold is critical to a molders success.
About the Author
| Steve Gerbig, DSM Engineering Plastics Headquarters Americas P.O. Box 3333 2267 West Mill Road Evansville, IN 47732-3333 Phone: 800-333-4237 Fax: 812-435-7702 Website: http://www.dsm.com/corporate/home.html |
With 40 years of experience in the plastics industry, Steve Gerbig has collected a vast amount of expertise in the area of engineering plastics. Since 1981, Steve has worked within DSM Engineering Plastics as a technical service engineer, market development specialist, product manager, and currently the technical support manager.Prior to DSM, Steve worked as both product and tool design engineer, sales engineer, manufacturing manager and general manager of several molding companies. Steve graduated from the University of Evansville, IN. Over the years he has presented papers at SPE Annual Technical Conference (ANTEC) and SPI Plastic Parts Innovation Conference, and other national technical conferences. Steve has written the chapter on reinforced thermoplastics for the Encyclopedia of Polymer Science and Engineering, and had papers published in numerous technical journals including Plastics Engineering and Design News. |
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